Several factors may cause misregistry of an electron beam with a phosphor element on a color picture tube screen. One of these factors is the thermal expansion of a shadow mask of the tube, when the mask is heated by electron beams from an electron gun of the tube that strike the mask. The shadow mask is usually attached to a peripheral frame that surrounds the mask. During tube operation, heat from the mask flows into the frame, creating a differential in temperatures between the center and peripheral portions of the mask. Because of this differential, the mask center, mask periphery and frame expand at different rates. These different expansion rates result in an arching or doming of the shadow mask. Because of such doming, the electron beams passing through the mask misregister with the phosphor elements of the tube screen. One method of compensating for mask doming is taught in U.S. Pat. No. 4,136,300, issued to A. M. Morrell on Jan. 23, 1979. That patent discloses the desirability of increasing the curvature of a mask to reduce electron beam misregister caused by mask doming. The patent also teaches that, with the increased curvature, the horizontal center-to-center spacing between shadow mask apertures should be increased from the center of the mask to the ends of the horizontal axis.
In the design of dot screen type color picture tubes that can be used in video displays, it is desirable to utilize greater mask curvature along with variable aperture spacing, in order to gain the advantage of reduced misregister as well as the additional advantages of being able to use higher anode power, providing simpler manufacturability, increased mask strength and reduced microphonics. However, a problem exists, relating to how aperture spacing should be varied in order to obtain a screen with uniformly straight parallel rows of phosphor dots, to minimize moire.